Carbon fibers are the main high strength reinforcing material used in fabrication of high performance composite materials. Strength-to-weight properties of carbon fiber reinforced composites (CFC) are superior to any other materials that are bringing about the revolution in many industrial areas such as construction, aviation, space, etc. In general, carbon-based composite materials comprise carbon fibers and a matrix. Different materials, such as polymers, carbons, ceramic, metals, glass, etc. could be used as a matrix in composite materials. The matrix must have the ability to transfer stress between fibers so that all the fibers used are effective in bearing the load. However one of the major problems associated with CFC materials relates to the weak interlaminar strength and bonding between the carbon fiber and matrix (especially an inorganic matrix). This could potentially lead to failure due to delaminating of the plies and/or fiber pull-out in CFCs.
Numerous attempts have been made to improve bonding between a fiber and a matrix consisting mostly of chemical and physical modifications to the surface of the fiber [L. Peebles, Carbon Fibers: Formation, Structure and Properties. CRC Press, Boca Raton, 1994]. For example, according to one approach the fiber surface was etched by oxidizing agents [P. Ehrburger, In Carbon, Fibers, Filaments and Composites, (Ed. J. Figueiredo et al.) Kluwer Academic Publ., Dotrecht (1989)]. The advantages were two-fold: firstly, the surface of fiber was roughened and increased, and, secondly, polar functional groups were introduced, which also enhanced the adhesion of fiber to the matrix. More recently, electrochemical etching [C. Kozlowski, P. Sherwood, Carbon, v. 24, 357 (1986)] and plasma [L. Drzal, M. Rich, P. Lloyd, J. Adhesion, v.16, p.1 (1982)] etching, as well as reaction with atomic oxygen [P. Pattabiraman, N. Rodrigues, B. Jang, R. Baker, Carbon, v. 28, p.867 (1990)] have also been used to increase bonding between the fibers and the matrix. These methods, however, could potentially lead to extensive damage and weakening of the structure [P. Pattabiraman, N. Rodrigues, B. Jang, R. Baker, Carbon, v. 28, p.867 (1990)]. In another approach, silicon carbide (SiC) whiskers were grown from the surface of a carbon fiber [J. Milewski et. al., U.S. Pat., No. 3,580,731 (1971)]. That process involved chemical vapor deposition (CVD) of SiC at temperatures above 1400xc2x0 C. This method, however, suffers from a number of shortcomings, related mostly to the differences in the density and the thermal expansion coefficients of SiC and carbon fiber, and difficulty of handling such an abrasive material.
An attempt to produce carbon fiber structures suitable for use in high performance composites by growing carbon filaments on the surface of primary carbon fibers (PCF) via a catalyzed CVD technique was reported [R. Baker et al, U.S. Pat. No. 5,413,866 (1995), and W. Downs and R. Baker, Carbon, v.19, No. 8, pp.1173-1179 (1991)]. The presence of carbon filaments enhances the interfacial bonding between the fiber and the matrix, which greatly reduces the problems associated with the delaminating of the composite. The concept is based on decomposition of selected hydrocarbons, preferably, ethylene (in a mixture with hydrogen), on the hot metal surfaces, preferably, Nixe2x80x94Cu alloy (70:30). During this reaction, growth of carbon filaments are influenced by several factors including: (a) the catalyst particle determines the morphology, the diameter, and the degree of crystallinity of graphitic units in the filament; and (b) during the filaments growth, the hydrocarbon is adsorbed and decomposed on the metal catalyst particle, followed by the diffusion of carbon species through the catalyst particle and the precipitation at the back of it, producing the filament structure. In general, the catalyst particle is located at the growing end of the filament, and is carried away from the surface of the support. Catalytic filament growth ceases when the leading face of the catalyst particle is encapsulated by a layer of carbon, which prevents further hydrocarbon decomposition. The filaments growth via catalyzed CVD occurs at 600xc2x0 C. with the typical diameters of filaments varying from 5 mn to 1000 nm (or 1 xcexcm), and the lengths from 5 to 100 xcexcm. The method suffers from the following disadvantages:
1) the method is very complex and multi-step as it includesxe2x80x94(i) the impregnation of PCF by aqueous solutions of mixed metal salts, (ii) the calcination in an oxidizing environment (air) to convert metal salts to metal oxides, (iii) reduction of metals oxides into metals using hydrogen-helium mixture, (iv) decomposition of ethylene (in a 50-50 mixture with hydrogen) over metal catalyst particles;
2) during the impregnation stage, the catalyst tends to accumulate in the crevices between the adjacent fibers, which leads to the predominant growth of filaments in these areas, and, as a result, to a non-uniform distribution of filaments;
3) the filaments are relatively thin ( less than 1 xcexcm) and predominantly curly which would result in a relatively weak micromechanical interaction with the matrix, particularly, metal and ceramic;
4) the method does not provide means for producing a protective coating for PCF; and,
5) due to intrinsic complexity of the method, it would be very difficult to arrange a continuous process.
A similar chemical vapor deposition technique was used by the authors [P. McAllister, E. Wolf, Carbon, v.30, No.2, pp.189-200 (1992)] to perform catalytic chemical vapor infiltration as the means of improvement of carbon-carbon composites. The authors grew carbon filaments on carbon fibers using Ni-catalyst particles and propylene as a source of carbon for the filaments. The method suffered from the same drawbacks recited as numbers 1-5 in the foregoing and additional drawbacks.
It is apparent from the above discussion that the bonding between the carbon fiber and the matrix in most cases is improved via increase in the micromechanical interaction between the fiber and the matrix which is directly proportional to the interfacial surface area. All the prior art systems offer rather limited capabilities for increasing the interfacial surface area between the carbon fiber and the matrix. While the existing methods for increasing the bonding between the carbon fibers and the matrix do improve the interlaminar strength of composite materials, there is a need for novel carbon fiber materials with the increased surface area and improved micromechanical interaction with the matrix. Furthermore, the 3D carbon fibers with the increased surface area can find a wide application in other areas, such as, adsorbents, catalyst supports, fuel cells, capacitors, medicine, refrigeration, environmental control and others.
It is a primary objective of the invention to provide novel high performance 3D carbon fibers comprised of original (or primary) carbon fibers (OCF) with the secondary carbon filaments (SCF) grown there from, and the tertiary carbon filaments (TCF) grown from the surface of SCF forming three-dimensional high surface area carbon structure.
Another objective of the invention is to develop novel methods and apparatus for the production of 3D carbon fibers by means of thermal decomposition of carbonaceous gases (CG) (or vapors) in an oxidant-free atmosphere over the heated surface of OCF and, if necessary, SCF.
Another object of the invention is to develop a method and apparatus for the growth of SCF on the surface of OCF by thermal decomposition of CG including, but not limited, to saturated, unsaturated and aromatic hydrocarbons, over the surface of resistively heated OCF (or other means of heating).
A further object of the invention is to develop a method and apparatus for growing SCF of different thickness, from 100 nm to 100 xcexcm, and length, from 1 xcexcm to 1 cm, by varying the operational conditions of the process, e.g., the nature of CG used, additives to CG, temperature of OCF and temperature gradient in the reactor, partial pressure, CG residence time, etc.
Another object of the invention is to develop a method and apparatus for the growth of tertiary carbon filaments (TCF) on the surface of heated SCF by thermal decomposition of CG including, but not limited, to saturated, unsaturated and aromatic hydrocarbons and carbon monoxide, in the presence of metal catalyst particles.
An additional object of the invention is to cover the surface of OCF with the protective carbon (pyrocarbon) coating to prevent oxidative, corrosive or other possible chemical or mechanical damages to the OCF.
Another object of the invention is to further increase the surface area of 3D carbon fibers by the treatment of the surface of OCF and SCF with activating agents (e.g., steam, CO2 and others).
In preferred product embodiments of the invention, the structure of 3D carbon fibers is OCF modified with surface grown predominantly straight SCF and if desired curly TCF grown on the SCF surface. The former embodiment is better described as three-dimensional (3D) carbon fibers comprising secondary carbon filaments (SCF) grown on the surface of the original carbon fibers (OCF) and characterized by a catalytic metal weight content of less than approximately 100 parts per million. These (SCF) are from approximately 100 nm to approximately 100 xcexcm thickness and have lengths of from approximately 1 xcexcm to approximately 1 cm. The latter embodiment is better described as three-dimensional 3D carbon fibers comprising tertiary carbon filaments (TCF) grown as branches on the surface of the secondary carbon filaments (SCF) and characterized by a catalytic metal weight content in excess of approximately 0.1 weight percent. The (TCF) are from approximately 10 nm to approximately 1 xcexcm thickness and have lengths of from approximately 1 xcexcm to approximately 100 xcexcm.
In one of the preferred method embodiments of the present invention, 3D carbon fibers were prepared by thermally decomposing a carbonaceous gas in an oxidant-free atmosphere over the surface of heated original carbon fibers (OCF); and, recovering OCF with secondary carbon fibers (SCF) on its surfaces thereby realizing three-dimensional (3D) carbon fibers which for increased area can be activated by exposure to steam or CO2.
In a second preferred method embodiment, the SCF modified OCF has deposited active metal catalyst particles on the surfaces of the recovered secondary carbon fibers (SCF) and thereafter subjected to a carbonaceous gas thermally decomposed in an oxidant-free atmosphere over the surface of the heated SCF whereby tertiary carbon filaments (TCF) are grown on the SCF surfaces.
In the preferred apparatus embodiment, the apparatus for producing three-dimensional (3D) carbon fibers comprises: a first chamber for growing secondary carbon filament (SCF); means for supplying original carbon filament (OCF) into said first chamber; means for introducing carbonaceous gas (CG) into and out of said first chamber; means for heating said OCF to a decomposition temperature for said CG; means for controlling the temperature and the time of exposure of said heated OCF to said CG within said chamber until the desired SCF growth is realized and, means for recovery of said SCF is provided.
Further objects and advantages of this invention will be apparent from the following detailed descriptions of presently preferred embodiments which are illustrated schematically in the accompanying drawings.